This invention relates to a method and apparatus for degreasing thin sheets of material and in particular to the removal of processing oil from the pores of sheets of microporous plastic material.
Material of this type has many uses, a typical one being the envelopes which contain the plates in lead acid storage batteries. The composition of such material is described in detail in Wayne, U.S. Pat. No. 3,351,495. The material is made from a mixture of plastic, usually a high molecular weight polyolefin, a filler, such as silica, talc, calcium carbide or carbon black, and a plasticizer, typically a petroleum-based oil. The mixture has a high percentage of plasticizer, typically 70 percent by weight, which is dispersed throughout the material in microscopic veins. After mixing, the heated mixture is extruded and calendered to produce a thin sheet which is cooled to room temperature in order to harden it. Lastly a substantial portion of the plasticizer is extracted from the sheet leaving pores which give the finished material its desirable characteristics.
Since approximately 50 percent of the total weight of the sheet is removed as oil, and the oil is distributed throughout the material in a network of extremely small diameter pores, the removal of the oil is not a simple matter. Heretofore oil has been extracted from this type of material by immersing in a liquid solvent. As the oil is removed from the material the solvent becomes soiled and, as a result, less effective. Therefore, the prior art devices used multiple tanks with solvent flowing from one tank to the next in the opposite direction that the material moves through the tanks so that the first tank the material passes through has the highest percentage of oil in it and each succeeding tank has a lower concentration of oil. However, since liquid extraction is a slow process large tanks are required and large quantities of solvent must be used. This not only makes the cost of the extraction system very high but also requires a large floor area for the tanks. In addition, large tanks have large surface areas thereby causing large quantities of the solvent to evaporate. Finally, the large volumes of the solvent which are used means that the energy cost for recovery of the oil from the solvent is high since the solvent must be evaporated to achieve separation.
For the foregoing reasons, the prior art degreasing systems are extremely expensive to build and to operate, are very inefficient of material and energy, and cause high levels of pollution. What is needed therefore is a way to remove the processing oil from thin microporous material which overcomes the shortcomings and limitations of the prior art systems.